1. Field of the Invention
The present invention relates to a slurry supply system for a Chemical Mechanical Polishing (CMP) process used in manufacturing semiconductor devices, and more particularly, to a slurry supply system for supplying the slurry to the CMP process facility in a state where the slurry is devoid of clustered primary abrasive particles, using a sound wave generating device to break apart the secondary abrasive particles.
2. Description of the Related Art
As semiconductor devices become more highly integrated, more sophisticated patterns, and advanced techniques for forming such patterns, are required. Present semiconductor devices are layered structures comprised of multiple insulating and conductive layers, with complex patterns being formed thereon to achieve the required circuit distribution.
However, the surface topology of the semiconductor device layers is becoming more complicated, and the step-height difference between layers can cause malfunctions in subsequent fabrication process steps. In addition, the aspect ratio of contact holes formed in the layers is increasing, which also causes fabrication and processing difficulties.
Among the various fabrication processes, photolithography is used to form a photoresist pattern on a semiconductor substrate. The photolithography process involves: coating a wafer with photoresist; aligning a pattern mask on the wafer, where the pattern mask has a specific circuit pattern formed thereon; exposing the photoresist on the wafer by irradiating light through the pattern mask; and developing the photoresist to form the photoresist pattern.
In early semiconductor devices, the critical dimension for feature resolution was relatively wide and the device itself was comprised of only a few layers. Accordingly, the photolithography process did not cause many problems. However, in present devices, the critical dimension is much narrower and the devices have numerous layers, which makes it increasingly difficult to achieve precise exposure focus between the upper and lower layers of the step-height differences among the layers. Therefore, it becomes more difficult to form a photoresist pattern having a precise critical dimension and vertical profile.
Accordingly, there is a great demand for improved wafer planarization techniques that address the processing problems caused by the increased aspect ratio and the step-height differences of the present semiconductor devices.
Planarization techniques to decrease the step-height difference include deposition of a spin-on-glass (SOG) layer, etch back methods, reflow methods, and global planarization methods.
Global planarization methods are performed along the whole surface of the wafer, and one such technique, Chemical Mechanical Polishing (CMP), planarizes the surface of the wafer through combined chemical and physical mechanisms.
During a CMP process, slurry is supplied between the wafer surface having circuits pattern formed thereon and the surface polishing pad confronting the wafer surface. While the slurry and the wafer surface are reacting chemically, the wafer and the polishing pad rotate in different directions relative to each other, such that protrusions or projections along the wafer surface are polished and the surface of the wafer is planarized.
The removal rate and uniformity are important factors in the CMP process, and these factors depend on several considerations including the CMP facility process conditions, the types of slurries employed, and the types of polishing pads that are used. In particular, the slurry composition, its pH, and the ion concentration, have a great impact on the resulting chemical reaction with the thin film being planarized.
Slurry compositions are generally of two types: oxide film slurry and metal film slurry. The oxide film slurry is alkali, and the metal film slurry is acidic.
For example, when silicon dioxide (SiO.sub.2) is planarized using an oxide film CMP process, the reaction with the alkali slurry causes the silicon dioxide (SiO.sub.2) to become hydrophillic, such that is readily absorbs water. The water induced into the silicon dioxide (SiO.sub.2) disconnects the bonds between the silicon dioxide (SiO.sub.2) atoms, and the silicon dioxide (SiO.sub.2) is then removed by the physical mechanism (friction) between the rotating pad and wafer, together with an abrasive.
On the other hand, when a metal layer is planarized using a metal film CMP process, a chemical reaction on the surface of the metal film caused by an oxidant inside the slurry creates a metal oxide film. The metal oxide film is then removed by the physical mechanism (friction) between the rotating pad and wafer, together with an abrasive.
More specifically, the metal film slurry comprises an oxidizing agent, an abrasive, deionized water, and acid. The abrasives in the slurry are composed of so-called primary abrasive particles, having a diameter ranging from about 130 nm to about 170 nm.
The conventional slurry supply system is constructed such that the slurry is continuously circulated through a line connected to a slurry tank in order to prevent the slurry from stagnating and thereafter deteriorating inside the slurry tank. While the slurry is circulating, some of the slurry is tapped off and discharged by means of a pump, and supplied to a pad table of the CMP process facility.
FIG. 1 shows a conventional slurry supply system of the CMP process for manufacturing semiconductor devices. Generally, the system supplies a slurry 2 stored inside a slurry tank 1 to the CMP facility through a slurry supply line 4. The slurry 2 inside the slurry tank 1 is pumped out of the slurry tank 1 using a pump (not shown), is circulated through a slurry discharge line 3a and a slurry recirculation line 3b, before reentering the slurry tank 1. At a point along the slurry discharge line 3a and the slurry recirculation line 3b, there is connected a slurry supply line 4 and a pump 5 along the slurry supply line 4 for tapping off some of the slurry 2. The slurry 2 is then provided to the pad table 6 of the CMP facility via a nozzle 7, so that the planarization process for the wafer 8 secured by a wafer holder 34 can be easily performed.
However, sometimes the abrasives contained in the slurry undesirably cluster in such a manner as shown in FIG. 2. More specifically, the abrasives in the slurry should preferably exist in the state of the sole primary abrasive particles 9, but despite the continuous circulation of the slurry 2, some of the primary abrasive particles 9 tend to cluster chemically or physically to thereby form larger so-called secondary abrasive particles 10. The secondary abrasive particles 10 may have diameters of 330 nm to 570 nm or more, as compared to diameters of 130 nm to 170 nm for the primary abrasive particles 9. These primary abrasive particles 9 and secondary abrasive particles 10 remain mixed in the slurry 2 and are supplied to the pad table 6. The larger secondary abrasive particles 10 may cause fine scratches on the wafer surface during the CMP process.
Such fine scratches on the wafer surface can thereafter induce non-uniform deposition of layers during the photoresist coating process or Chemical Vapor Deposition (CVD) process, thereby producing cut-line defects in the metal layers.
Prior methods to prevent abrasive particle clustering have included minimizing the particle size of the abrasive, adding chemicals such as a surface-active agent, or preventing the congestion and dryness of the supplied slurry. However, the generation of the secondary abrasive particles has not been completely prevented due to the various chemical characteristics of the components of the slurry, and so, there fine scratches on the wafer surface still present a significant processing problem.